In a past year, apparently I didn't check off that all of my HSA expenses were qualified expenses. How do I go about amending that?

The IRS probably wants a form 8889 filled in ...   

To enter medical expenses in TurboTax, please follow these steps:

1. Click on Federal Taxes > Deductions & Credits  [In TT Self-Employed:  Personal > Deductions & Credits > I'll choose what I work on].
2. In the Medical section, click on the Start/Revisit box next to Medical Expenses.  
3. Continue through the interview, entering the requested information on your medical expenses.
4. You will be asked to confirm the amount you withdrew from your HSA/MSA to pay for medical expenses. 

Thank you!